1. Field of the Invention
The present invention relates to a technique of controlling a switching regulator having outputs in a plurality of channels.
2. Description of the Related Art
A secondary battery such as a lithium ion battery is mounted on various electronic apparatuses of recent years such as a portable telephone terminal, a Personal Digital Assistant (PDA), or a notebook-type personal computer. A lithium ion battery generates a battery voltage of about 3 to 4 V in accordance with a charged state. However, electronic devices such as a microprocessor operating at a power source voltage of 1.5 V or lower or a light-emitting diode operating at about 5 V are mounted on the electronic apparatuses. In order to give a suitable power source voltage to these devices, one makes use of a switching regulator that raises or lowers a voltage of the battery voltage.
Japanese Patent Application Laid-open (JP-A) No. 2003-289666 and International Publication No. 2006/043370A1 Pamphlet disclose a technique of providing a switching power supply apparatus that can generate a plurality of output voltages at a low cost with saved space. This technique aims at reducing the number of components by sharing the inductor and the main switch with a plurality of output voltages in a switching regulator for outputting a plurality of direct current voltages.
The inventor of the present invention propose a technique of constructing a transistor for synchronous rectification with two transistors that are connected in series, and disposing the transistors so that the body diodes of the two transistors will be in opposite directions (See International Publication No. 2006/043370A1 Pamphlet). The International Publication No. 2006/043370A1 Pamphlet discloses a controlling method of simultaneously turning on the two transistors that are connected in series and changing the on-time of one of the two transistors in accordance with the pulse width (duty ratio) of a pulse modulation signal.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2003-289666
[Patent Document 2] International Publication No. 2006/043370A1 Pamphlet
When there is a time difference until the transistor for synchronous rectification is turned on after the main switch is turned off, the connection point of the inductor and the main switch (which is referred to as a switching terminal) will be in a high-impedance state, and the electric potential thereof swings largely, thereby raising a problem.
When the voltage of this switching terminal (which is referred to as a switching voltage) becomes an overvoltage, there is a fear that the loss of the circuit will be large, or the switching voltage exceeds the breakdown voltage of the transistor to affect the reliability of the circuit.